Power save multi-poll
The last 802.11-based power-saving mechanism covered here is Power Save Multi-Poll (PSMP). This mechanism (Figure 6) is advertised as a way for VoIP clients to save power, but it does so only in some circumstances. PSMP trades the overhead of listening to the PSMP beacon (and potentially listening through some voice packets destined for other nodes) for less contention on the medium when a large number of VoIP devices are trying to operate at the same time.

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Figure 6: VoIP being carried by PSMP.

PSMP emulates a TDMA system within the larger 802.11 system. The PSMP beacon essentially carves out time for a TDMA mode of operation. The beacon gets all other devices to be silent after they hear it, and it carries a schedule for when the AP is going to send downlink packets to the PSMP-capable devices—different VoIP phones, for example. The AP also uses the beacon to schedule these devices' uplinks.

Contrasting PSMP with APSD is interesting. In the latter mode, the device is only spending its time sending uplink traffic and getting the ACKs, or receiving downlink traffic and sending the ACKs. It is very efficient.

In the case of PSMP, a device has to wake up and receive the PSMP beacon, which is pure overhead. Depending on how many other wireless devices are in the network, and the timing of the activities, the device may not have time to transition into the sleep mode, wake back up, and get a packet.

It may be that the gap in time is short enough that the device must stay awake while other devices receive their VoIP packets. If so, that time is also wasted overhead. The same kind of situation can happen with the transmit period of PSMP operation.

Yet another PSMP inefficiency occurs because the wireless channel can vary rapidly. As devices move around, the supportable data rate increases and decreases. As a result, PSMP devices need to be conservative and use a lower data rate because failed packets are a burden on the system.

If a packet fails in a TDMA system, on the other hand, the system has to make up for that failure at a much later time. The system has to allocate a new time slot to retransmit the packet, because the whole idea behind TDMA is to pack transmissions right next to each other. There is no time allowed for retransmitting failed packets.

In contrast, with APSD, a packet might fail, but it can be retransmitted immediately, without centralized coordination. This makes the penalty for a failed packet much lower, and allows APSD devices to use higher data rates more aggressively, reducing power consumption (Figure 7).

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Figure 7: VoIP being carried by APSD.

All of these factors cause PSMP to have a significant amount of overhead, so this mode only pays off if the number of VoIP nodes associated with an AP exceeds approximately 15. Under APSD, the devices wake up according to their own timing and are not coordinated. With that many devices contending for the airwaves, they start colliding or waiting for each other to finish their transmissions.

As a rule of thumb, with a modest number of VoIP devices, APSD is the best approach. A network with a large number of VoIP calls (>15) going simultaneously to the same access point might benefit from PSMP.